1. Field of Invention
The present invention relates to a picture image processing method which processes a picture image of an original medium that has been optically input.
2. Description of Related Art
A picture image input apparatus (e.g., a scanner) optically inputs and photoelectrically converts a picture image of an original medium, and outputs this picture image as electronic data. In this kind of scanner, when the picture image of an original medium is input, a lookup table is necessary in order to reproduce the object image. This look-up table is, for example, one in which a table used to correct variances characteristic of the apparatus and a table used for picture image adjustment according to commands from the user are merged.
In the picture image input of a negative original medium (negative film), which is a transmissive original medium, the negative gradation conversion table used to convert the negative to a positive is also merged into the look-up table. The negative film has light and dark gradation or coloration opposite that of the object.
Furthermore, in the picture image input of a negative film, it is necessary to perform a prescan in order to create this negative gradation conversion table. This prescan performs the same actions as the normal scan during picture image input, with the exception of using a default look-up table to create a look-up table.
This negative gradation conversion table creation is started, for example, by commands from a host computer to which the scanner is connected.
First, the scanner receives from the host computer the resolution, the picture image input range and the data in the look-up table into which the negative gradation conversion table of linear properties which is the basis is merged. The scanner, upon receiving from the host computer a command to start scanning, then starts the prescan.
Next, the scanner creates a histogram of the brightness of the input picture image and the frequency thereof from the picture image data input through the prescan.
A shadow point used to match the white point is found from this histogram. The white point is the location where the value of the output of the negative gradation conversion table is a maximum. In addition, the shadow point is the darkest location on the negative film, and is the brightest location in the original object. That is to say, the white point is the brightest location in the original object, and is the point which should be the brightest location in the data from the negative-to-positive conversion of the picture image which was input.
The negative gradation conversion table shown in FIGS. 5A and 5B is created using the shadow points and highlight points found as described above. FIG. 5A shows the basic negative gradation conversion table which is the basis of the initial state when picture image input of the negative film is accomplished. In addition, FIG. 5B shows the negative gradation conversion table transformed by the results of the above-described prescan.
If n is the number of bits in the negative gradation conversion table, the negative gradation conversion table has an output value that is an integer in the range from 0 to N for an input value in the range from 0 to N, where N=2.sup.n -1.
In the basic negative gradation conversion table shown in FIG. 5A, the data which is obtained by photographing a gray scale with a commonly known change in density using a predetermined negative film and inputting this into the scanner is set so as to reproduce the density change of the original gray scale.
Accordingly, this basic negative gradation conversion table is one which provides an output value "out" for a given input value "in", and can be approximately defined as shown in equation (1) below. EQU out=f(in) (1)
Here, f(0)=N, and in addition, f(N)=0.
By performing white point matching for each of RGB following this basic negative gradation conversion table, equation (2) below results. EQU out=f(in)=N, (0.ltoreq.in &lt;S) EQU out=f((in-S.sub.R).multidot.N/(N-S.sub.R)) EQU out=f((in-S.sub.G).multidot.N/(N-S.sub.G)) EQU out=f((in-S.sub.B).multidot.N/(N-S.sub.B)), (S.ltoreq.in&lt;N)(2)
As shown in FIG. 5B, the negative gradation conversion tables 51, 52 and 53 having S.sub.R, S.sub.G, and S.sub.B as the shadow points result. In FIG. 5B, H is the highlight point. The highlight point is the brightest location on the negative film, but is the darkest location in the original object.
In the scanner, the negative gradation conversion table obtained in this manner is merged with the look-up table, and the picture image data input through the actual picture image input is converted using this and is transferred to the host computer.
However, when the shadow point of the negative gradation conversion table is caused to coincide with the white point of the negative film as described above, the gradient of the negative gradation conversion table becomes more steep the higher the value of the shadow point.
For example, as shown in FIG. 5B, the shadow point S.sub.G of G is a higher value than the shadow point S.sub.R of R. Consequently, the negative gradation conversion table 52 has a steeper gradient than the negative gradation conversion table 51.
When the negative-positive conversion is accomplished using the negative gradation conversion tables, the post-conversion value becomes lower than the shadow point the higher the pre-conversion value.
Consequently, the brightness under the conversion by the negative gradation conversion table 52 having a steep gradient falls more severely than the brightness under the conversion with the negative gradation conversion table 51.
As a result, white is reproduced as white, but the brightness of G is lower than that of R and B in gray regions farther from white, so that the problem arises that the picture image which has been converted has a purplish red hue.